Someone said that courage is just fear that a set its prayers. But we wondered if courage is also place in the mind or in the heart so ABC's Dan Harris went looking for the answers to all -- -- this week in Texas and Boston. As a nation was shaken and brilliance fire. In Boston this week America bore witness to -- college student who after running 26 miles. Literally gave the shirt off his own back to treat a victim's wounds. The grieving father who'd come to hand out flags in honor of his dead sons who left into the fray. And the off duty firefighter who gave aid to a badly injured boy wanted to do it. You just do it. You really glad you're there you can do your -- that make a difference. In west Texas overnight similar tales of heroism William birch pulled the injured. Out of -- demolished nursing home you've got to be able to reach out and help the ones that are and not able to help themselves what causes somebody to run toward danger when others are running away. Scientists say having a connection to the victims like a boyfriend who shielded their girlfriends in the a -- -- movie theater massacre. Or to a place as in Boston and in Texas. Can make a difference but ultimately says this Harvard professor it is a rare mysterious quality that is nearly impossible to predict none of us know whether right. We have a -- -- time. Until we -- faced with a crisis situation that puts extraordinary demands on ordinary people. Traditionally mankind has been viewed as essentially selfish survival of the fittest and all that but in fact. There willingness to risk our own lives to help -- may be what has allowed us to survive as a species. And certainly to endure disasters like we've seen this week. Dan Harris ABC news Boston.

This transcript has been automatically generated and may not be 100% accurate.